Gas is becoming more and more attractive fuel for ships' and other marine vessels' prime movers and auxiliary engines. Particularly but not exclusively natural gas (NG) is feasible due to its availability. Natural gas is a gaseous mixture in the ambient circumstances consisting primarily of methane and small amounts of ethane, propane, butane and nitrogen. It has high hydrogen content relative to coal, so when combusted it provides inter alia low amount of emissions, very clean burning process and it is basically free of contaminants. Particularly in cruise vessels, ferries and so called ropax vessels, where passengers are on board, the absence of soot emissions and visible smoke in the exhaust gases of ship's engines is very important feature facilitated by using NG as fuel for the engines but also for vessels carrying goods and bulk materials. Usually natural gas is stored as liquefied natural gas (LNG) at temperature of about −162° C., thus the storage cause problems particularly when the LNG is stored at high pressure, about at 5 bar, which is typically the level that a gas operated piston engine requires.
When bunkering i.e. filling a fuel bunker of a LNG fuelled vessel the pressure inside the vessel's LNG tank is usually needs to be lowered to facilitate the flow of liquefied gas from a storage tank to the vessel's LNG tank in order to have a minimal/low enough counter pressure in the vessel's tank compared to the pressure in the storage tank i.e. LNG bunker tank. The lowering of the pressure may be accomplished by spraying cold LNG from the top of the tank which cools down the gas in the gaseous phase decreasing the pressure.
However, after bunkering the pressure must be again rebuilt in order to have a sufficient gas pressure for feeding the gas to the engines of the vessel at appropriate pressure. In the prior art systems the pressure is rebuilt by heating up the LNG and returning the gas at the top of the tank by means of a so called pressure build up circuit.
If the engines of the vessel are not running or are only idling there is only a limited heat available after bunkering. This means that the pressure build up takes a long time.
In WO2011053164A1 there is shown an LNG fuel tank system for at least one gas engine used for ship propulsion, comprising at least one LNG fuel tank and a gas vessel, the LNG fuel tank to be bunkered from an onshore LNG pressure tank filling facility by means of a LNG filling line, wherein the LNG fuel tank is a ship low pressure controlled atmospheric pressure LNG tank, and the gas vessel is a single shell non-insulated pressure vessel arranged to accumulate flashed and boil-off gas during LNG bunkering and pressure relieving the LNG fuel tank, respectively. The gas engines are fuelled from either the gas vessel or the LNG fuel tank, dependent on a predefined gas vessel pressure. Thus, the publication suggests accumulating flashed and boil-off gas during LNG bunkering while maintaining the pressure in the LNG fuel tank low by means of a compressor connected between the LNG fuel tank and the gas vessel. Fuelling the gas engine of the vessel is taken place so that gas from the gas vessel is used until the pressure approaches about 5-6 bar gauge, where after the fuelling is effected from the LNG fuel tank.
Although the fuel system shown in WO2011053164A1 may be advantageous as such, there has recently come out demands to provide other solutions to bunkering LNG fuel tank in a marine vessel particularly in a situation where the LNG tank(s) of the vessel is/are pressure vessel(s).
It is an object of the invention to provide method of filling a fuel tank with liquefied gas, and a liquefied gas system, which provides more simple operation and immediate utilization of the gas as fuel for a gas operated piston engine in a marine vessel.